In a percussion drill string, the cutting bit may be attached to a percussion sub at the bottom of the drill string, while most of the rest of the drill string can be repeatedly raised and dropped to hit the percussion sub to aid in drilling. If the top of the percussion sub is repeatedly hit, then some of the force is dissipated during its travel through the percussion sub to the cutting bit. Also, damage can occur to portions of the percussion sub. If the hammering occurs at the lower portion of the percussion sub, then there is a possibility that cuttings from the earth will find their way between the hammering surfaces and damage them. A percussion sub which effectively transmitted hammering forces to the cutting bit while avoiding damage to the hammering surfaces, would be of considerable value.